The present invention relates to a combustion control method and device for an internal-combustion engine.
More particularly, it relates to the combustion of a homogeneous mixture of fuel with the intake air or with a mixture of air and recirculated exhaust gas (EGR).
Reduction of the emissions generated by internal-combustion engines, combined with a reduction of the consumption, is of constant concern to internal-combustion engine developers.
In new engine generations, for low and medium loads, the combustion of the fuel mixture, conventionally ignited either by compression or by a plug, is to be replaced by an ignition by compression of a homogeneous charge type combustion.
This is notably known for Diesel engines as  less than  less than (Homogeneous Charge Compression Ignition greater than  greater than  (HCCI) and, for gasoline engines, for a combustion mode called  less than  less than Controlled Auto Ignition greater than  greater than  (CAI).
This combustion is obtained when the homogeneous mixture of fuel, air and possibly recirculated exhaust gas has reached a certain temperature threshold which favours its autoignition combustion.
Generally, this type of combustion is characterised by a stage of slow oxidation of the fuel mixture prior to the combustion of this mixture.
This type of combustion allows to reduce considerably the nitrogen oxides (NOx) and particles emissions discharged by the engine.
However, this combustion involves a certain number of drawbacks which are by no means insignificant.
In fact, it is necessary to obtain a good combustion timing, i.e. to determine the exact moment when the fuel mixture ignites, so that this combustion occurs notably when the position of the piston has reached its appropriate position at the time when the fuel mixture has all the physico-chemical characteristics required for autoignition.
Many means exist to this end, such as feeding steam into or using recirculated exhaust gas in the combustion chamber.
The most commonly used means to obtain the required timing consists in feeding into or in keeping in the combustion chamber exhaust gas from the engine, referred to as recirculated exhaust gas, with a high proportion so as to time the combustion at the desired time.
This recirculated exhaust gas allows the combustion to be  less than  less than calmed down greater than  greater than  as it reduces the oxygen content (O2) of the fuel mixture.
However, these high proportions of recirculated exhaust gas lead to high fuel/air ratios that are close to stoichiometric conditions, which reduces all the more the operating range of the engine. In fact, beyond a fuel/air ratio above 0.96, the fuel mixture no longer contains enough oxygen to burn properly.
Furthermore, because of these high fuel/air ratios, emissions such as carbon oxide (CO), unburnt hydrocarbons (HC) and fumes are of high amplitude.
The present invention aims to overcome the aforementioned drawbacks by providing a combustion control method allowing to obtain the desired combustion timing.
To this end, a method of controlling the combustion by autoignition of a homogeneous fuel mixture for an internal-combustion engine comprising a combustion chamber in which a stage of slow oxidation of said mixture occurs prior to the combustion of this mixture is characterised in that a determined amount of fuel is fed into the combustion chamber during the slow oxidation of the fuel mixture so as to lengthen this oxidation stage.
Advantageously, the determined amount of fuel can be introduced in at least one fuel injection.
Preferably, an amount of fuel less than or equal to 10 mm3 can be introduced.
This amount of fuel can range between 1 and 8 mm3.
Preferably, a fuel mixture consisting of fuel, air and recirculated exhaust gas can be prepared in the combustion chamber.
A fuel mixture can be prepared with a maximum exhaust gas recirculation ratio of the order of 0.7.
The fuel introduced can be of gasoline or Diesel type.
The invention also relates to a device for controlling the combustion by autoignition of a homogeneous fuel mixture for an internal-combustion engine comprising a combustion chamber in which a stage of slow oxidation of said mixture occurs prior to the combustion of the fuel mixture, characterised in that it comprises means for controlling the injection of a determined amount of fuel during the stage of slow oxidation of this mixture.
The control means can be the engine computer which controls the fuel injectors.